As a result of the inherent degeneracy associated with the polarization of light traveling in an optical fiber, it is often important to be able to measure the state of polarization (SOP) of an optical signal at a given point in time and space. Polarization measurement is important, for example, in accurately orienting polarization maintaining fiber during a splicing operation, in measuring the polarization dependent loss (PDL) of components or systems, as well as in determining the polarization-mode dispersion (PMD) in optical transmission systems. Polarization mode dispersion is an increasingly critical phenomenon that will need to be controlled with high precision in the emerging high-speed (multi-Gbps), long-haul systems.
Most prior art arrangements for measuring polarization mode dispersion rely on a statistical sampling of polarization states. See, for example, U.S. Pat. No. 5,440,390 issued to Tirri et al. on Aug. 8, 1995. As networks grow increasingly complex, especially with the advent of local access, optical signal monitoring will become more important to ensure reliable operation. Real-time measurement of polarization mode dispersion and correlation of this data with bit-error rate (BER) or system quality would be a useful tool in measuring the optical system performance. Moreover, some processes could be improved, or at least made more deterministic, if a simple in-line technique were available for providing knowledge of the exact state of polarization within the transmission fiber.
An exemplary prior art "in-line" fiber optic polarimeter is described in an article entitled "Fiber-optic four-detector polarimeter" by A. Bouzid et al. appearing in Optics Communications, Vol. 18, 1995, at pp. 329-324. In the Bouzid et al. arrangement a set of four externally-induced, in-core fiber gratings are used in association with four photodetectors to perform the polarization measurements. The article purports to provide for the measurement of all four Stokes parameters simultaneously by measuring the intensity of light absorbed (or reflected or transmitted) by four photodetectors at four different planes of incidence. However, the arrangement lacks any phase retardation element, which is necessary in order to accurately define the required Stokes parameters. Additionally, this polarimeter lacks sufficient bandwidth to be useful in telecommunications applications.
Thus, a need remains in the art for an in-line polarization measurement device that is readily available for use in systems, particularly where it could be combined with a polarization controller to enable polarization-sensitive devices or functions to be utilized.